It is known that two-part isocyanate-based adhesives can be used to bond natural and synthetic materials including paper, metals and plastics. The two-part adhesives generally consist of polyols and polyisocyanates, often mixed immediately before the bonding process, which cure to form polyurethane-based adhesives. Several references are described below:
U.S. Pat. No. 3,901,852 (Shah) discloses polyurethane-based elastomers comprising the reaction product of 4,4'-methylenebis (phenyl isocyanate), polyethylene glycols and aliphatic straight chain diols. The elastomers are formed using a one-shot procedure in which all the reactants are brought together simultaneously before the bonding process. Shah has found that by controlling the proportion of polyethylene glycol to extender within very carefully defined limits and by employing certain extenders in combination with the polyethylene glycols, it is possible to obtain thermoplastic polyurethanes that possess all the desirable structural strength properties of those elastomers currently available. U.S. Pat. No. 3,901,852 (Shah) fails to teach or suggest two-part isocyanate-based adhesive compositions, using a one-shot procedure, which phase-separate during the curing process to form highly-opaque white polyurethanes.
U.S. Pat. No. 4,532,316 (Henn) discloses phase separating polyurethane prepolymers and elastomers prepared by reacting a polyol having a molecular weight of 600-3500, isocyanate and a low molecular weight chain extender wherein the ratios of reactants have an isocyanate index of greater than or equal to 1.1. The prepolymers phase separate and form a highly-opaque white solid having excellent storage stability. Henn further describes a method for the preparation of polyurethane elastomers wherein the prepolymers are reacted with low molecular weight chain extenders. In the most preferred embodiment, the prepolymers of the invention are prepared by slow addition of polyol to liquid diisocyanate in such a manner as to favor a narrow molecular weight distribution of isocyanate terminated prepolymer, followed by the addition of the deficiency of chain extender. The two-part isocyanate-based adhesive compositions require a complicated and costly pre-formation of a solidified prepolymer which requires a melting process prior to its application.
German Patent application DE 42 08 862 A1 (Boehnke) discloses a filter element and a process for its production wherein the adhesive is preferably a hot melt adhesive, but can also be a two-component or a one-component adhesive; materials exemplified are polyamide or polyurethane adhesives. This reference does not disclose any details of how the two-component polyurethane adhesive is formed or any of its physical properties.
Other related patents which describe the preparation of filters include: U.S. Pat. No. 4,210,067 (Evans, Jr.); U.S. Pat. No. 4,227,953 (Wasielewski etal.); and U.S. Pat. No. 4,976,857 (Solomon).
In the manufacture of bonded filters, it is desirable to have an adhesive which closely matches the white appearance of the filter media. Adhesives having a highly-opaque white appearance are often formed by the addition of inorganic pigments and/or fillers. Examples of commonly used inorganic pigments and/or fillers include titanium dioxide, antimony oxide, zinc oxide, magnesium silicate (talc), calcium sulfate, barium sulfate, zinc borate, anhydrous sodium potassium alumina silicate, calcium sulfate and calcium carbonate. Such materials are not, however, desirable for the adhesives' appearance and processing characteristics. Specifically, these materials often settle, prior to the meter-mix process, making the adhesive appear less uniform in color. Although sedimentation may be controlled by the addition of thickening agents, such additives can further adversely affect the flow characteristics, resulting in reduced fiber penetration and bond strengths. Another disadvantage associated with white adhesives having inorganic pigments and/or fillers relates to the adhesives' abrasive characteristics in that such materials often wear down mechanical parts within the meter-mix and/or dispensing equipment.
The present inventors have now discovered a two-part polyurethane adhesive system that possesses a white-opalescent appearance without the addition of any filler or pigments.